The present invention relates to cabin pressure control systems and, more particularly, to a cabin pressure control system having a dual valve control and monitoring architecture.
Aircraft cabin pressure control systems are necessary to maintain controlled cabin pressures when flying at high altitudes, where ambient air pressure is reduced. Above 10,000 feet ambient air pressure becomes low enough to cause passengers to suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
However, at altitudes above 10,000 feet, such as 43,000 feet and above, if the cabin pressure inside an aircraft were not permitted to decrease, the pressure difference between inside cabin pressure and outside ambient pressure can become sufficiently great to cause a catastrophic rupturing of the aircraft. Accordingly, it has been standard practice to permit cabin pressure to decrease to a value corresponding to an altitude of about 8,000 feet. Thus, structural integrity of the aircraft can be maintained while providing adequate oxygen for passenger breathing.
Large variations in cabin pressure can damage or destroy the aircraft fuselage. Variations in cabin pressure must also be controlled for the sake of passenger safety and comfort. Since the human ear is more sensitive to increases in pressure (descent in elevation) than to decreases in pressure (ascent in elevation), the passenger comfort factor is complicated by the need for different permissible maximum rate changes. Furthermore, for maximum passenger comfort the cabin pressure should not be subject to spikes or changes when the aircraft momentarily climbs or drops in altitude.
In general, electric cabin pressure control systems that meet these requirements include an outflow valve that controls the pressure differential between actual pressure in a cabin and the surrounding atmosphere. The outflow valve receives a drive signal from a controller and a driver. The controller calculates an output signal based on the pressure differential between the cabin and the atmosphere and additional critical parameters. This output signal actuates the outflow valve to keep the actual cabin pressure near a predetermined control cabin pressure.
A malfunction of the outflow valve or the controller may cause the pressure differential between the cabin pressure and the atmosphere pressure to exceed a predetermined threshold. In case of a positive pressure differential (cabin pressure higher than atmosphere pressure) a safety valve may open mechanically based on this pressure differential. Likewise, in the case of a malfunction that results in a negative pressure differential (cabin pressure lower than atmosphere pressure), a negative relief valve may allow entry of air into the cabin.
Because of the potential serious consequences of undesirable cabin pressure changes, current aircraft safety regulations require a high level of redundancy in cabin pressure control systems. Unfortunately, redundancy usually requires extra components such as outflow valves, control motors, and controllers, which add to the weight, space, installation costs and maintenance cost of cabin pressure control systems.
As can be seen, there is a need for an aircraft cabin pressure control system that is highly reliable, has adequate redundancy and which minimizes the added weight, space, installation cost and maintenance cost of the system.